As the center for oxidative phosphorylation and apoptotic regulation, mitochondria play a vital role in human health. For example, mitochondrial dysfunction has been linked to cancer, Alzheimer's disease, and congestive heart failure among many other diseases. The protein quality control network for the outer mitochondrial membrane (OMM) is essential for proper mitochondrial function as it removes old, damaged, or mislocalized proteins from the OMM while leaving resident OMM proteins unperturbed. Msp1 was recently identified as the retrotranslocase for mislocalized TA proteins in the OMM. Msp1 knockdown in human cell culture results in mitochondrial fragmentation, loss of mitochondrial DNA, and severe defects in oxidative phosphorylation, while knockout mice die a few weeks after birth. Despite this, virtually nothing is known about how Msp1 recognizes and extracts mislocalized TA proteins from the OMM. Here, I propose a series of experiments to address fundamental questions regarding Msp1 function. In Aim 1, I use X-ray crystallography, yeast genetics, and biochemistry to build a mechanistic framework for how Msp1 extracts substrates from the OMM. In Aim 2, I use in vitro reconstitution to examine how Msp1 recognizes substrates for extraction, with a particular focus on Bcl-2 family members. Together, these experiments will shed light on how Msp1 maintains OMM proteostasis and open new possibilities for therapeutic intervention in a variety of human diseases.